1. Field of the Invention
The present invention relates to a heating apparatus in a microwave oven in which one of microwave oven elements, namely a tray in a cavity where a food article to be cooked is usually placed is used as a medium of an ultrasonic oscillator.
2. Discussion of the Background Art
In general, a microwave oven cooks a food article positioned in a cooking chamber by application of heat energy from an electric heater as a heat source. Another auxiliary heat source, magnetron for example, is sometimes installed to use microwaves as a separate heat source.
FIG. 1 is a schematic perspective view illustrating a main part of a related art microwave oven.
As shown in FIG. 1, the microwave oven includes a case 1; a cavity 2 formed inside of the case 1; a door 3 pivotably fastened to the entire low surface of the case 1 to be able to open/close the cavity 2; an operation display 5 installed at a top front surface of the cavity 2, displaying an operation state of every button needed for operation of the microwave and displaying a state of the oven; a lower heater 7 installed between a low portion of the cavity 2 and the case, providing heat through the bottom of the cavity 2; an upper heater 9 installed at an upper portion of the cavity 2, heating a food article inside the cavity with radiant heat from the heater; a convection heater 11 installed between the bottom surface of the cavity 2 and the case 1; a convection fan 13 installed at the bottom surface of the cavity 2, supplying heated air by the convection heater 11 into the cavity 2; a cooling fan 17 installed between the upper portion of the cavity 2 and the case 1, cooling electric elements; an oven lamp 18 for illuminating an inside of the cooking chamber; and a magnetron 15 for generating microwaves for cooking the food article.
On the top end of the door 3 is a knob 4. Thus, when a user wants to open the cavity 2, he or she needs to pull the knob 4 installed at the top end of the door 3. What happens then is the door is rotated by means of a hinge connecting the lower end of the case 1 with the door 3, and the closed cavity 2 is opened.
Also, to supply heated air by the convection heater 11 into the cavity 2 through an operation of the convection fan 13, a plurality of air passage holes 14 are perforated on the bottom surface of the cavity 2 opposed to the convection heater 11.
FIG. 2 is a state diagram illustrating a food article positioned inside of the related art microwave oven's cavity being cooked by heat from a heater and by magnetron.
An operation of the related art microwave oven with the above configuration is now described.
When a user places a food article at an inside of the cavity 2 and closes the door 3, the cavity 2 starts cooking the food article while the cavity 2 is being shut by the door 3. At this point, heat generated from the lower heater 7, which is installed between the lower portion of the cavity 2 and the case 1, is conducted to the bottom surface of the cavity 2, and the conducted heat is eventually transmitted to the food article through air circulation inside of the cavity 2 and through a tray where the food article is placed.
Moreover, heat generated from the upper heater 9, which is installed at the upper portion of the cavity 2, is transmitted to the food article through radiation and convection. Further, by operation of the convection fan 13, heat from the convection heater 11 is also transmitted in form of hot wind to the food article via the plurality of air passage holes 14 formed on the bottom surface of the cavity 2.
However, the related art microwave oven poses a serious problem in its heating method. For example, a part of the food article is cooked by heat from the upper heater 9 installed at the top end of the cavity 2 through radiation and convection, as shown in FIG. 2. In this case, since an inside temperature of the cavity 2 is forcefully mixed by the convection fan 13, the inside of the cavity 2 can have a homogenous temperature field overall. However, there is a limit to get the heat to be effectively absorbed by the food article placed at the tray 19 though the above heating method. Particularly, when the heater 9 is used to cook the food article, heat is not evenly reached the inside of the food article, but only the surface area of the food article is cooked, resulting in overcooking or burning the surface of the food article.
To prevent the surface of the food article from burning by heat from the heater 9, manufacturers made the heater 9 to be turned on/off as needed. However, it turned out that this turning on/off method only prolonged the cooking time more than necessary.